1. Field of the Invention
The present invention relates to a method for forming a metal layer having irregular structures in a simpler manner and a method for fabricating a light emitting device with improved light extraction efficiency that uses a metal layer formed by the above method.
2. Description of the Related Art
Light emitting devices are semiconductor devices in which an electrical current is allowed to flow in the forward direction through a PN junction to generate light.
Light emitting devices using semiconductors convert electrical energy into light energy with high efficiency, have a service life as long as 5 to 10 years, and possess the advantages of low power consumption and greatly reduced maintenance and repair costs. Due to these advantages, light emitting devices have received attention for applications in next generation lighting devices.
Sapphire substrates are typically used for the growth of gallium nitride-based compound semiconductors in the fabrication of light emitting devices. A general low-power gallium nitride-based light emitting device is fabricated in such a manner that a sapphire substrate, on which a crystal structure has grown, is positioned on a lead frame and two electrodes are connected to each other on the top of the substrate. For the purpose of improving the heat dissipation efficiency of the device, the thickness of the sapphire substrate is reduced to about 80 microns or less before bonding to the lead frame. Considering that the thermal conductivity of the sapphire substrate is about 50 W/m⋅K, the sapphire substrate suffers from very high thermal resistance even when its thickness is reduced to about 80 microns, making it difficult to obtain desired heat dissipation characteristics.
Under such circumstances, flip-chip bonding is used to further improve the heat dissipation characteristics of high-power gallium nitride-based light emitting devices. Flip-chip bonding is a technique in which a chip with a light emitting diode structure is flip-bonded to a highly thermally conductive submount such as a silicon wafer (thermal conductivity ˜150 W/m⋅K) or an AIN ceramic substrate (thermal conductivity ˜180 W/m⋅K). In this case, heat is dissipated through the submount substrate. This heat dissipation improves the thermal dissipation efficiency compared to heat dissipation through a sapphire substrate. However, the improvement does not reach a satisfactory level and the fabrication process is complicated.
In order to solve such problems, recent attention has been directed toward sapphire substrate-free vertical LEDs. Vertical LEDs can be fabricated by removing a sapphire substrate from a light emitting structure using a laser lift-off (LLO) technique before packaging. The laser lift-off technique makes the fabrication process simpler than flip-chip bonding. Vertical LEDs are known as structures with the best heat dissipation efficiency.
The emission area of a LED fabricated by a flip-chip bonding technique is about 60% of the chip area, while that of a sapphire substrate-free vertical LED reaches 90% of the chip area. Accordingly, the sapphire substrate-free structure exhibits better characteristics.
Despite these advantages, however, the sapphire substrate-free vertical LED exhibits lower light extraction efficiency than conventional light emitting devices. The reason for this is as follows. The sapphire substrate-free LED structure is covered with a molding material such as epoxy or a molding material mixed with a phosphor. At this time, a large difference in refractive index between gallium nitride (GaN) and the molding material causes total reflection of a considerable portion of light from the LED structure without being emitted to the outside. The reflected light returns back to the light emitting structure and evanesces. Assuming that the refractive index of the molding material is about 1.5, the amount of total reflected light from the interface between the molding material and gallium nitride (refractive index ˜2.6) is about 9%. Thus, further improvement in light extraction efficiency is needed.
To meet this need, research has been conducted on methods in which a sapphire substrate is removed and irregularities are formed on the exposed surface of a p-type gallium nitride layer before or after electrode wiring. For example, a technique using evanescent waves generated by total reflection is disclosed in Appl. Phys. Lett. 94, 091102 (2009). According to this technique, a ridge structure is allowed to grow on a V-grooved substrate to form a light emitting structure and constructive coupling of evanescent waves occurs depending on the design of the structure, achieving improved light extraction efficiency. However, further research is still needed to sufficiently improve the light extraction efficiency of light emitting devices.